The first time I spoke with Sofie I didnt realise she was an orphan child, and I didnt have a house with a free room to adopt her at the time, so I cancelled the conversation and planned to come back after getting a new place. Since going back to her I no longer have the option to adopt her - when she stands at the dock gates, my only speech option is for her wares, and when she is around town I can only ask her about her dead parents. I currently have Lucia and one of the boys from the orphanage - does adopting from there perhaps interfere with your ability to adopt her?

You can only adopt two kids in the unmodded game. Since you're on the playstation, you can't use mods.